Inversion (pixel-walk)

One of the eleven boxes below may flicker slightly. The a/b
variants are the same, except that they depend on the position of your
browser window. Strong flicker indicates that the voltages in the
monitor are misadjusted. Unfortunately, this is probably not
something you can adjust by yourself. In practical situations, a
visible flicker here means that plain colors show "pixel walk" when
watched from close-by.

Both inversion tests below require that the monitor is in its native
resolution. If the monitor is on a VGA (not DVI) cable, the
clock and phase settings (test page) settings should be
correct.

You can also try the full-screen inversion below, but be warned
that the flickering might be disturbing. Then click "next" and watch
the screen. Most LCD monitors seem to be sensitive to patterns 4a or
4b. The Asus Eee triggers on pattern 7a/b. As a side effect, some
other patterns may display a ghost shadow to the right of the black
navigation rectangle. An ideal monitor does not flicker and does not
show this ghosting either. In the full-screen test, the flickering may
be stronger close to the edges of your monitor compared to the center
of the screen.

Background

[click to show flickering image]
Flickering example.

(Pixel walk simulation, zoomed in 500%)

In a pixel on an LCD monitor, the amount of light that is
transmitted from the backlight depends on the voltage applied to the
pixel. For the amount of light, it doesn't matter whether that voltage
is negative or positive. However, applying the same voltage for a
long period would damage the pixel. Do you remember how electricity
decomposes water into oxygen and hydrogen gas in chemistry class?
Similar things could happen inside the liquid crystals that are in the
pixels. In order to prevent damage, LCD displays quickly alternate the
voltage between positive and negative for each pixel, which is called
'polarity inversion'. Ideally, the rapid polarity inversion wouldn't
be noticeable because every pixel has the same brightness whether it a
positive or a negative voltage is applied. However, in practice, there
is a small difference, which means that every pixel flickers at about
30 hertz. In order to make this less noticeable, pixels with positive
and negative voltages are interleaved, such that on average the screen
as a whole keeps the same brightness – at least for normal
images. The interleaving does not work for
the inversion test images, at least for the
one(s) where the pattern matches the interleaving pattern of the
monitor. In such cases, the monitor will flicker if the positive and
negative voltages are not tuned very well. Note that the patterns 2,
4, 6, and 7 are split into an 'a' and a 'b' version. The only
difference between the 'a' and 'b' versions is that they are offset
vertically (2,4,6) or horizontally (7) by one pixel; which one
actually flickers is dependent on how you position your browser
window.

The top example image flickers at about 12 Hz, a bit dependent on
your browser settings. The test patterns are likely to flicker faster,
at 30 Hz. In practice, you will not see a flicker this strong for
normal images. However, you might see 'pixel walk' in certain
colors. Pixel walk looks like in the image on the right

This pattern is a simulation; the real pattern would be 5 times
smaller and move with a speed of about 1.5 cm per second (3/4 inch per
second), either leftwards or rightwards. Below is a plain colored
surface where you can try to see the pattern. Pixel walk is best
visible when you watch your monitor from close-by (less than 15 cm or,
6 inch) and you follow the train of yellow dots with one eye, keeping
the other eye closed. The actual pixel walk can be slightly slower or
faster than this. The stronger the flickering is in the test patterns,
the more noticeable the pixel walk will be. Note that the flickering
in the test patterns often is stronger near the edges of the screen,
so you may need to move your browser window.

The idea of using these dot patterns to visualize the polarity
inversion was originally by William Andrew Steer. More information on
this topic is
on Techmind.org and in a
separate whitepaper by another
author: LCD
screens don't flicker – or do they? Intersil application
note AN1208.0 (2005).